# How do you solve the quadratic equation by completing the square: #x^2 + 4x - 21 = 0#?

##### 1 Answer

#### Explanation:

Your starting quadratic equation looks like this

To solve this quadratic by completing the square, you need to add a term to both sides of the equation, such that the left side of the equation becomes equivalent to the **square of a binomial**.

First, move the term that does not contain

To do that, you need to divide the *coefficient* of the **2**, **square** it, then add the result to both sides of the equation.

In your case, you have

Your quadratic will become

The left side of the equation can be wrtitten as

This means that you have

To solve this equation, take the square root from both sides of the equation to get

The two solutions for this equation are